Mo-Cu composite powder

ABSTRACT

A Mo—Cu composite powder is provided which is comprised of individual finite particles each having a copper phase and a molybdenum phase wherein the molybdenum phase substantially encapsulates the copper phase. The composite powder may be consolidated by conventional P/M techniques and sintered without copper bleedout according to the method described herein to produce Mo—Cu pseudoalloy articles having very good shape retention, a high sintered density, and a fine microstructure.

TECHNICAL FIELD

[0001] This invention relates to molybdenum-copper (Mo—Cu) compositepowders and in particular to Mo—Cu composite powders used to makecomponents for electronics, electronic packaging, and electricalengineering applications. Examples of such applications include heatsinks, thermal spreaders, electrical contacts, and welding electrodes.

BACKGROUND ART

[0002] Mo—Cu pseudoalloys possess properties that are similar to theproperties of W—Cu pseudoalloys. However, they have the additionaladvantages of lower weight and higher workability which makes thembetter suited for miniaturized electronics.

[0003] One conventional method for making articles comprised of Mo—Cupseudoalloys consists of infiltrating separately sintered porousmolybdenum blanks with liquid copper. Infiltrated articles have a solidmolybdenum skeleton that functions as the backbone of the pseudoalloy.The skeleton retains the liquid copper during infiltration (and hightemperature operation) by capillary forces. One drawback of theinfiltration method is that it does not allow near-net or net-shapefabrication of parts. Hence, a number of machining operations arerequired to obtain the final shape of the infiltrated article.

[0004] Other conventional methods for forming Mo—Cu articles includeconsolidating blends of molybdenum and copper powders by powdermetallurgical (P/M) techniques such as hot pressing, explosive pressing,injection molding, tape forming, and rolling. Unlike the infiltrationmethod, these methods do not have a separate step for sintering amolybdenum skeleton. As a result, articles made by P/M methods eithercompletely lack a molybdenum skeleton or have a skeleton of reducedstrength. High compacting pressure, repressing, resintering, andsintering under pressure (hot pressing) have been suggested to improveMo—Mo contacts and the strength of the Mo skeleton. Although P/Mtechniques allow near-net or net-shape fabrication, sintering articlesto full density is complicated by the lack of solubility in the Mo—Cusystem, poor wetting of molybdenum by copper, and by copper bleedoutfrom parts during sintering. Furthermore, additions of sinteringactivators such as nickel and cobalt to improve densification aredetrimental to the thermal conductivity of Mo—Cu pseudoalloys, aproperty which is critical for a number of electronics applications.

[0005] In order to improve the homogeneity and density of Mo—Cupseudoalloys made by P/M methods, Mo—Cu composite powders have been usedwherein the molybdenum particles have been coated with copper bychemical deposition or electroplating. However, the copper coatingreduces the contact area between molybdenum particles and the strengthof the molybdenum skeleton. Moreover, these powders do not preventcopper bleedout from parts during sintering, and hot pressing is stillrequired to improve the sintered density of articles. Thus, it would beadvantageous to have a Mo—Cu composite powder which could be used in P/Mmethods to form net or near-net shaped Mo—Cu articles having strongsintered molybdenum skeletons without copper bleedout.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to obviate the disadvantages ofthe prior art.

[0007] It is another object of the invention to provide a Mo—Cucomposite powder with a phase distribution that facilitates theformation of a strong molybdenum skeleton and internal infiltration ofthe skeleton with liquid copper during sintering.

[0008] It is a further object of the invention to provide a Mo—Cucomposite oxide powder for producing a Mo—Cu composite powder having ahigh level of mixing of the metal phases.

[0009] It is still a further object of the invention to provide a P/Mmethod of making Mo—Cu pseudoalloy articles with a strong molybdenumskeleton and a high sintered density without copper bleedout.

[0010] In accordance with an object of the invention, there is provideda molybdenum-copper composite powder comprising individual finiteparticles each having a copper phase and a molybdenum phase wherein themolybdenum phase substantially encapsulates the copper phase.

[0011] In accordance with another object of the invention, there isprovided a method of making a CuMoO₄-based composite oxide powdercomprising:

[0012] (a) forming a mixture of a molybdenum oxide and a copper oxide,the molybdenum oxide being selected from ammonium dimolybdate, ammoniumparamolybdate, or molybdenum dioxide; and

[0013] (b) firing the mixture at a temperature and for a time sufficientto form the CuMoO₄-based composite oxide.

[0014] In accordance with still another object of the invention, theMo—Cu composite powder of this invention is made by the methodcomprising:

[0015] (a) reducing a CuMoO₄-based composite oxide powder in a firststage to form an intimate mixture of metallic copper and molybdenumoxides without the formation of low-melting-point cuprous molybdatephases; and

[0016] (b) reducing the intimate mixture in a second stage at atemperature and for a time sufficient to reduce the molybdenum oxides tomolybdenum metal.

[0017] In another aspect of the invention, there is provided a methodfor making a Mo—Cu pseudoalloy comprising:

[0018] (a) consolidating a Mo—Cu composite powder to form a compact, theMo—Cu composite powder having a copper content from about 2 wt. % toabout 40 wt. % and comprising individual finite particles each having acopper phase and a molybdenum phase wherein the molybdenum phasesubstantially encapsulates the copper phase;

[0019] (b) sintering the compact in a first sintering stage at atemperature from about 1030° C. to about 1050° C. to form a molybdenumskeleton;

[0020] (c) sintering the compact in a second sintering stage at atemperature from about 1050° C. to about 1080° C. for a compact madefrom a composite powder having a copper content of about 26 wt. % toabout 40 wt. %, or at a temperature from about 1085° C. to about 1400°C. for a compact made from a composite powder having a copper content ofabout 2 wt. % to about 25 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an x-ray diffraction pattern of a first stage reductionproduct formed by the hydrogen reduction at 300° C. of a CuMoO₄-basedcomposite oxide powder having a relative copper content of 15 wt. %.

[0022]FIG. 2 is an SEM micrograph of a cross section of an agglomerateof the Mo—Cu composite powder taken using Back-Scattered ElectronImaging.

[0023]FIG. 3 is an enlargement of the finite particle outlined in themicrograph shown in FIG. 2.

[0024]FIG. 4 is an SEM micrograph of a cross section of a Mo-15Cupseudoalloy.

[0025]FIG. 5 is an SEM micrograph of a cross section of a Mo-40Cupseudoalloy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] For a better understanding of the present invention, togetherwith other and further objects, advantages and capabilities thereof,reference is made to the following disclosure and appended claims takenin conjunction with the above-described drawings.

[0027] We have invented a molybdenum-copper (Mo—Cu) composite powderwhich comprises finite dual-phase particles each having a copper phaseand a molybdenum phase wherein the copper phase is substantiallyencapsulated by the molybdenum phase. The bulk Mo—Cu composite powdersof this invention have the gray color of unalloyed molybdenum powderswhich is consistent with the substantial encapsulation of the copperphase by the molybdenum phase. Preferably, the Mo—Cu composite powderscontain from about 2 wt. % to about 40 wt. % Cu.

[0028] In general, the as-reduced Mo—Cu composite powders consist oflarger agglomerates (on the order of about 15 μm to about 25 μm in size)of the finite dual-phase particles. Under SEM examination, the finiteparticles within the agglomerates are shown to be irregularly shaped andhave a size in the range of about 0.5 μm to about 1.5 μm. Each of thefinite particles has a sintered molybdenum network in which the voidsare filled with copper. This unique distribution of Mo and Cu phasesprovides substantial encapsulation of the Cu phase by the Mo phase andresults in the highest level of mixing within the larger agglomerates.

[0029] Because the copper phase is encapsulated by the molybdenum phase,an enhanced sintering process with several desirable features isachieved. These features include: (1) the formation of Mo—Mo particlecontacts after pressing the powder, (2) the sintering of a substantiallydense Mo skeleton prior to melting of the copper, (3) internalinfiltration of the skeleton with liquid copper and retention of copperwithin the skeleton by capillary forces, and (4) sintering in thepresence of liquid copper without copper bleedout from the compact.

[0030] The Mo—Cu composite powders are produced by the chemicalsynthesis and hydrogen reduction of cupric molybdate-based compositeoxide systems having controlled amounts of molybdenum trioxide (MoO₃).Generally, cupric molybdate (CuMoO₄) is made by a solid phase reactionbetween MoO₃ and CuO or Cu₂O in air at 600° C. for 40 hours. However,the copper metal content of CuMoO₄ as a percentage the total metalcontent (Cu+Mo) is quite high, about 40 weight percent (wt. %). This ismuch higher than the copper content in Mo—Cu pseudoalloys used in someindustrial applications. For instance, a copper content of 15 to 25 wt.% is required in Mo—Cu materials for electronic packaging. This problemis solved by transitioning the copper content of the CuMoO₄-basedcomposite oxide over a wide range by co-synthesis of varied amounts of asecond phase of MoO₃. The combination of the CuMoO₄ and MoO₃ phaseslowers the copper content of the composite oxide into the ranges desiredfor electronics applications. Preferably, the copper content of theCuMoO₄-based composite oxide as a percentage of the total metal contentmay be varied from about 2 wt. % to about 40 wt. %. Unless otherwiseindicated, the relative copper content for the composite oxides isexpressed herein as a percentage of the total metal content of theoxide.

CuMoO₄-based Composite Oxide Synthesis

[0031] The composite oxides were produced by solid phase synthesis. Theratio of solid reactants (copper and molybdenum oxides) was adequate tosynthesize end products containing the CuMoO₄ phase and a controlledamount of the MoO₃ phase. The ratio of the synthesized phases(particularly, the amount of MoO₃) controlled the copper content in theCuMoO₄-based composite oxides. The general formula of the preferredCuMoO₄-based composite oxide may be represented on a mole basis asCuMoO₄+xMoO₃ where x is from about 29 to 0. The co-synthesized CuMoO₄and MoO₃ phases were present in every composite oxide particle whichprovided a very high level of mixing of the copper and molybdenum.

[0032] In the preferred synthesis methods, two combinations of reactantsare used: (i) complex molybdenum oxides with copper oxides, inparticular, ammonium dimolybdate (ADM, (NH₄)₂Mo₂O₇) or ammoniumparamolybdate (APM, (NH₄) 6 (Mo₇O₂₄ 4H₂O) with cuprous (Cu₂O) or cupric(CuO) oxides, and (ii) molybdenum dioxide (MoO₂) with cuprous or cupricoxide. At temperatures above 250° C. in air, the complex molybdenumoxides undergo thermal decomposition (e.g., (NH₄)₂Mo₂O₇→2MoO₃+2NH₃+H₂O),and Cu₂O and MoO₂ undergo oxidation to CuO and MoO₃. These phasetransitions dramatically increase the surface area and surface energy ofthe reactants which accelerates their solid phase interdiffusionreactions and the formation of CuMoO₄-based composite oxides. Thus, theCuMoO₄-based composite oxide may be formed by firing a mixture of theseoxides at a temperature from about 650° C. to about 750° C. for onlyabout 5 hours.

[0033] Silica was selected as the material of choice to contain thesolid phase synthesis of the CuMoO₄-based composite oxides becausemolybdenum trioxide wants to react and form molybdates with the majorityof other metals and metal oxides traditionally used to make boats andtrays for solid phase synthesis processes. The use of silica boatsslightly increased the silica content of the composite oxide compared tothe total silica content in the reactants. However, the total silicaremained at a low level and is not believed to substantially affect thesinterability of the final Mo—Cu composite powder or theelectrical/thermal conductivity of the Mo—Cu pseudoalloy.

[0034] The first preferred synthesis method may be illustrated by thereaction involving ADM and Cu₂O. The composition of ADM can berepresented as (NH₄)₂O.2MoO₃, and the synthesis reaction as:

0.5Cu₂O+n[(NH₄)₂O.2MoO₃)]+0.25O₂→CuMoO₄+(2n−1)MoO₃+2nNH₃ +nH₂O

[0035] By varying the factor n in the range of about 15.0 to 0.5, therelative copper content in the CuMoO₄-based composite oxides synthesizedby the above reaction may be controlled in the range of about 2 wt. % toabout 40 wt. %.

EXAMPLE 1

[0036] ADM with a median particle size of 198.8 μm and cuprous oxidewith a median particle size of 14.5 μm were used as solid reactants inthe synthesis of CuMoO₄-based composite oxides. (Unless otherwisespecified particle sizes were determined using a Microtrac MT, X-100particle size analyzer.) The total weight of solid reactants in thesetests was in the range of 0.5 to 1.0 kg. Blends were prepared in analumina ball mill. The weight ratio of alumina milling media to thereactants varied in the range of 1.5 to 1.0. The length of milling was 1hour. The light brown color of the milled blend of oxides was the resultof mixing the colors of ADM (white) and Cu₂O (brown). The use of millingis preferred with these reactants because the median particle size ofthe ADM and APM powders is generally much larger than that of the copperoxides which makes it difficult to obtain a homogenous blend bymechanical mixing alone.

[0037] Synthesis was carried out in air in a laboratory furnace with analumina tube. Silica boats were used as reaction containers. A load of150 g of the milled oxides produced a bed depth of about 0.5″ in theboat. The rate of the furnace temperature increase was 2° C./min. Thesynthesis temperature was 750° C. with an isothermal hold at thistemperature of 5 hours. Five tests were carried out in which therelative copper content in the synthesized composite oxide was varied inthe range of 8 to 40 wt. %.

[0038] All of the synthesized composite oxides formed sintered cakeswhich needed to be ground up in a mortar. The caking was attributed tothe high reaction temperature and high diffusion activity of the milledsolid reactants, particularly, the in-situ-produced MoO₃. The groundmaterials were screened −100 mesh. The synthesized powders had agreen-yellow color characteristic of the CuMoO₄-based composite oxides.Elemental and x-ray diffraction (XRD) phase analysis were used forpowder characterization. The major XRD peaks associated with the MoO₃(3.26 Å) and CuMoO₄ (3.73 Å) phases were used to calculate the XRD peakintensity ratios. Table 1 compares the calculated values for thecomposite oxides based on the amounts of the reactants with the measuredvalues for the synthesized composite oxides. TABLE 1 Calculated ValuesMeasured Values Relative Molar Ratio Relative XRD Peak Copper of CopperIntensity Ratio Content, wt. % MoO₃/CuMoO₄ Content, wt. % of MoO₃/CuMoO₄8 6.616 7.6 5.23 16 2.477 15.5 1.48 24 1.097 23.8 0.94 32 0.404 31.70.38 40 0 39.9 0.15

[0039] The correlation between the measured and the calculated values ofthe relative copper content increased with the copper content of thesynthesized powders and was in the range of 95.0% to 99.75%. A goodcorrelation was also observed between the trends for the predicted andthe actual ratios of the product phases versus the copper content.

[0040] In the second and more preferred synthesis method, a friableCuMoO₄-based composite oxide is made from unmilled dehydrated solidreactants, of which at least one reactant undergoes a phase change inthe course of synthesis, e.g., in-situ oxidation of MoO₂ to MoO₃ andCu₂O to CuO. As in the first method, the copper content of thesynthesized composite oxide is controlled by varying the reactionstoichiometry. The synthesis reaction uses one mole of CuO or a halfmole of Cu₂O. For Cu₂0, the reaction may be represented as:

0.5Cu₂O+nMoO₂+(0.5n+0.25)O₂→CuMoO₄+(n−1)MoO₃

[0041] By varying the factor n in the range of about 30.0 to 1.0, therelative copper content in the CuMoO₄-based composite oxides may becontrolled in the range of about 2 wt. % to about 40 wt. %. Also, sincethe median particle size of the molybdenum dioxide and copper oxides isof the same order of magnitude, homogeneous starting blends of the solidreactants could be made by mechanical mixing without milling.

EXAMPLE 2

[0042] In this example, MoO₂ (D₅₀=5.3 μm), Cu₂O (D₅₀=14.5 μm), and CuO(D₅₀=13.3 μm) were used to make the composite oxides. Starting blendswith a total weight of 0.5 to 1.0 kg were made by mixing the oxides in alaboratory V-blender for 30 min. The color of the starting blends rangedfrom brown (MoO₂ +Cu₂O) to dark brown (MoO₂+CuO).

[0043] The synthesis was carried out in air using the same hardware asin Example 1. A load of 100 g of the blended oxides produced a bed depthof the material in the boat of about 0.5 inches. The rate of furnacetemperature increase was 2° C./min. The synthesis temperature was 650°C. with an isothermal hold of 5 hours at this temperature. The relativecopper content in the synthesized composite oxide was varied in therange of 5 to 40 wt. %.

[0044] In each case, a uniform loosely sintered cake was formed. Thematerial was very friable, and could be disintegrated into powder byrubbing lightly between one's fingers. The synthesized powders had agreen-yellow color characteristic of the CuMoO₄-based composite oxides.Powders were screened −100 mesh and subjected to the same analysesdescribed in Example 1. The data in Table 2 illustrate the properties ofthe composite oxide powders synthesized from MoO₂ and Cu₂O (column A)and MoO₂ and CuO (column B). TABLE 2 Calculated Values Measured ValuesRelative XRD Peak Copper Molar Ratio Relative Copper Intensity RatioContent, of Content, wt. % of MoO₃/CuMoO₄ wt. % MoO₃/CuMoO₄ A B A B 511.583 4.68 4.66 7.2 7.2 10 4.961 9.64 9.60 3.2 4.3 15 2.753 14.45 14.471.9 2.3 20 1.649 19.30 19.49 1.2 1.8 25 0.987 24.39 24.27 0.8 0.9 300.545 29.52 29.62 0.5 0.3 35 0.230 34.53 34.66 0.1 0.3 40 0 39.40 39.780 0.1

[0045] The correlation between the measured and the calculated values ofthe relative copper content increased with the copper content and was inthe range of 93.4% to 99.0%. A good correlation is also observed betweenthe trends for the predicted and the actual ratios of the product phasesversus the copper content.

EXAMPLE 3

[0046] Additional tests were carried out to demonstrate that the coppercontent in the CuMoO₄-based composite oxides can be adjusted to thespecified level by controlling the amount of the copper oxideparticipating in the synthesis. An excess of 4 wt. % Cu₂O, compared tothe quantity required by the reaction stoichiometry, was used. Testconditions were exactly the same as in Example 2. Table 3 compares theactual copper content of the synthesized composite oxide with the coppercontent specified by stoichiometry. The correlation of the actual coppercontent with the specified values was calculated as a ratio of theactual to the specified copper content. TABLE 3 Specified Actual (NoActual (4 wt. % Corre- Cu, wt. % Excess Cu₂O) Correlation Excess Cu₂O)lation 5 4.68 0.936 5.08 1.016 10 9.64 0.964 10.39 1.039 15 14.45 0.96315.39 1.026 20 19.30 0.965 19.63 0.981 25 24.39 0.975 24.97 0.998 3029.52 0.984 29.75 0.991 35 34.53 0.986 35.76 1.022 40 39.40 0.985 40.531.013

[0047] These results demonstrate that adding an excess of up to 4 wt. %of the copper oxide reactant over that required by stoichiometry can beused to adjust the relative copper content in the resultant compositeoxide closer to the specified level.

EXAMPLE 4

[0048] The synthesis of a CuMoO₄-based composite oxide with a relativecopper content of 15 wt. % was performed in a production scale beltfurnace. The solid reactants were MoO₂ and Cu₂O. A 300 kg blend ofreactants with a 4 wt. % excess of Cu₂O was made in a production scaleV-blender. A 1.5 kg amount of the starting blend produced a material beddepth of about 0.5 inches in a silica tray. The synthesis was carriedout in air at an average temperature of 675° C. with an averageresidence time of about 4 hours. The furnace throughput was about 6 kgof composite oxide per hour. A total of 268 kg of the end product wassynthesized. The material was discharged from the silica tray onto avibrating screen, disintegrated, screened −60 mesh and collected in ahopper. The product was blended in a V-blender and analyzed for particlesize distribution and copper content. A sample of the end product wasmilled, screened −100 mesh and subjected to an XRD analysis. Thefollowing product characteristics were obtained:

[0049] Particle size distribution: D₉₀=18.5 μm

[0050] D₅₀=5.5 μm

[0051] D₁₀=2.1 μm

[0052] Relative copper content: 15.36 wt. %

[0053] XRD Peak intensity ratio of MoO₃/CuMoO₄: 1.8

[0054] The phase composition and copper content of the composite oxidepowder synthesized in the production furnace closely reproduced thecorresponding properties of powders synthesized in the laboratory.

Reduction of CuMoO₄-based Composite Oxides

[0055] One of the key problems which exists in the conventional methodsinvolving the co-reduction of mechanically blended mixtures of oxidepowders stems from the significant difference between the reductiontemperatures of the oxides of molybdenum and copper. This differencecauses a premature appearance of copper and its segregation bycoalescence. This in turn results in an inhomogeneous distribution ofthe Mo and Cu phases in the reduced Mo—Cu composite powders. Incontrast, the Mo—Cu composite powders produced by the hydrogen reductionof the synthesized CuMoO₄-based composite oxides exhibit very goodhomogeneity. The atomic level of contact between the copper andmolybdenum in the synthesized CuMoO₄-based composite oxides and thedifference in the reduction temperatures can be used to advantage bycontrolling the order of appearance of the metal phases therebyresulting in a homogeneous Mo—Cu composite metal powder comprised ofindividual dual-phase particles in which the Mo phase substantiallyencapsulates the Cu phase.

[0056] In a preferred method, the hydrogen reduction of the CuMoO₄-basedcomposite oxides is performed in two stages. The first stage reductionis performed at a temperature from about 250° C. to about 400° C. andcauses the reduction of copper from the composite oxides therebyyielding an intimate mixture of metallic copper and molybdenum oxides.The second stage reduction is performed at a higher temperature, fromabout 700° C. to about 950° C., and causes the reduction of themolybdenum oxides to molybdenum metal which results in the formation ofthe dual-phase particles and the substantial encapsulation of the copperphase by the molybdenum phase.

[0057] The two-stage reduction is preferred because the hydrogenreduction of the CuMoO₄ phase is complicated by disproportionation ofcupric molybdate into cuprous molybdates, Cu₆Mo₄O₁₅ and Cu₂Mo₃O₁₀, whichhave relatively low melting points (466° C. and 532° C., respectively).The formation of these phases at the initial stage of hydrogen reductionis detrimental as it fuses the powder and obstructs the reductionprocess. It was discovered that the formation of these liquid phases canbe prevented by taking advantage of the high thermodynamic probabilitythat the copper in the CuMoO₄-based composite oxides may be reduced attemperatures below the melting points of the cuprous molybdates. Usingthe lower reduction temperatures in a first-stage reduction eliminatesthe formation of the low-melting-point cuprous molybdates and producesan intimate mixture of Mo oxides and metallic copper. Although traces ofother molybdates, Cu₃Mo₂O₉ and Cu₆Mo₅O₁₈, have been identified asforming in the course of the first stage hydrogen reduction, thesemolybdates have a high temperature stability and do not cause anycomplications.

[0058] In the second stage, the molybdenum oxides are reduced tomolybdenum metal. The conventional method for reducing Mo from itstrioxide typically involves two steps which are carried out in differenttemperature ranges. First, MoO₃ is reduced to MoO₂ at 600-700° C. andthen the MoO₂ is reduced to Mo at 950-1100° C. However, in the reductionof CuMoO₄-based oxide composites, there appears to be a catalytic effectcaused by the freshly reduced Cu phase being in close contact with theMo oxides. This results in a lowering of the temperature of theMoO₃→MoO₂ reduction step to 350-400° C., and the MoO₂→Mo reduction stepto 700-950° C. In addition, the presence of the Cu phase leads to thedeposition of Mo on Cu which inhibits the coalescence and growth ofcopper particles and causes the gradual encapsulation of the Cu phase bythe Mo phase. This mechanism is believed to contribute to controllingthe size and homogeneity of the composite Mo—Cu particles.

[0059] After the second reduction stage, the as-reduced Mo—Cu compositepowders may require passivation to reduce their tendency towardoxidation and pyrophoricity. In particular, it was discovered thatoxidation and pyrophoricity of the as-reduced Mo—Cu composite powderswith an oxygen content below 5000 ppm may be suppressed by passivationof the powders for 1 to 2 hours with nitrogen immediately after removalfrom the furnace.

[0060] The following examples illustrate the reduction of thesynthesized CuMoO₄-based composite oxides to form the Mo—Cu compositepowders of this invention.

EXAMPLE 5

[0061] A first stage reduction of a CuMoO₄-based composite oxide havinga relative copper content of 15 wt. % was carried out in a laboratoryfurnace with flowing hydrogen. An oxide load of about 150 g produced amaterial bed depth of about 0.5 inches in the Inconel boat. Thereduction temperatures were 150, 200, 300, and 400° C. The rate offurnace temperature increase was 5° C./min, and the isothermal hold atthe reduction temperature was 4 hours. The resulting products werescreened -60 mesh and subjected to an XRD analysis.

[0062] No low-melting-point cuprous molybdates (Cu₆Mo₄O₁₅ or Cu₂Mo₃O₁₀)were detected in the reduction products. Minor Cu₃Mo₂O₉ and Cu₆Mo₅O₁₈phases were detected in products reduced in the 150-200° C. temperaturerange. The reduction of copper appears to begin at about 200° C. and iscomplete at about 300° C. The major phases in the material reduced at300° C. were Cu, MoO₂, and MoO₃. Apparently, underreduced MoO₃ in thematerial is quite active and undergoes an exothermic, partial hydrationwhen exposed to air. This required that the product reduced at 300° C.be cooled in air for about 20 to 30 min. Once cooled, the reducedproduct which had a dark gray color could be easily screened −60 mesh.The major phases of the product reduced at 400° C. were Cu and MoO₂. Inthis case, the product temperature did not increase after exposure toair. The product reduced at 400° C. was caked and requireddisintegration to turn it into a powder that could be screened −60 mesh.

EXAMPLE 6

[0063] A first stage reduction of the same CuMoO₄-based composite oxideused in Example 5 was performed in a large production-scale hydrogenreduction furnace having three heating zones. An oxide load of about 2kg produced a material bed depth of about 0.5 inches in an Inconel tray.In the first test, all zone temperatures were set at 300° C. In thesecond test, all zone temperatures were set at 400° C. The residencetime for the material in the furnace was about 4 hours. The end productswere screened −60 mesh and subjected to an XRD analysis.

[0064] After removal from the furnace, the temperature of the productfrom the first test increased requiring cooling in air for about30-45min. The major phases in the reduced product were Cu and MoO₂. Theminor phases consisted of various hydrated molybdenum trioxide phases.FIG. 1 illustrates the XRD pattern for this material. The appearance ofthe product closely resembled the material obtained in Example 5 at 300°C. The major phases in the reduced product from the second test were Cuand MoO₂. Traces of the hydrated molybdenum trioxide phases were alsopresent. The appearance of the product also closely resembled thematerial obtained in Example 5 at 400° C.

EXAMPLE 7

[0065] A two-stage hydrogen reduction of the synthesized CuMoO₄-basedcomposite oxides having a relative copper content in the range of 5 to40 wt. % was performed. The same hardware, loading conditions, and rateof temperature increase as in Example 5 were used. The reductiontemperatures were 300° C. (first stage) and 700° C. (second stage) witha four-hour isothermal hold at each temperature. After cooling thefurnace to below 200° C., the gas flow through the furnace tube wasswitched from hydrogen to nitrogen. The nitrogen flow was maintaineduntil the furnace cooled to about 30° C. This effectively passivated thereduced Mo—Cu composite metal powders. The bulk as-reduced powders had agray color similar to unalloyed Mo powders. There was no visualindication of the presence of copper in the Mo—Cu composite powders. Thepowders were screened −60 mesh and analyzed for Cu content, particlesize distribution, and surface area. The results of the differentanalyses are given in Table 4. TABLE 4 Mo—Cu Composite Powder PropertiesCu Median Surface CuMoO₄-based estimated, Cu actual, Size, Area,Composite Oxide wt. % wt. % μm m²/g  CuMoO₄ + 11.583 5 4.9 18.5 2.13MoO₃ CuMoO₄ + 4.961 10 10.2 19.7 1.53 MoO₃ CuMoO₄ + 2.753 15 15.3 15.71.95 MoO₃ CuMoO₄ + 1.649 20 20.4 23.5 1.61 MoO₃ CuMoO₄ + 0.987 25 25.922.3 (2.06) MoO₃ CuMoO₄ + 0.545 30 30.9 23.7 1.19 MoO₃ CuMoO₄ + 0.230 3536.2 25.1 1.44 MoO₃ CuMoO₄ ˜40 41.4 25.4 1.23

[0066] The measured copper content of the Mo—Cu composite powders wasgenerally about 2% to about 3.5% higher than the estimated value.

EXAMPLE 8

[0067] Tests were conducted on establishing the effect of thesecond-stage reduction temperature on properties of the resultant Mo—Cucomposite powders. The starting material was the CuMoO₄-based compositeoxide (15 wt. % Cu) which had been reduced at 300° C. in Example 6. Thesecond-stage reduction was carried out using temperatures in the rangeof 700° C. to 950° C. using the same conditions as in Example 7 exceptthe rate of temperature increase which was 20° C./min. Six reductionruns were carried out. The reduced Mo—Cu composite powders were screened−60 mesh and analyzed. The results of the analyses are presented inTable 5. TABLE 5 Reduction Temperature, ° C. Powder Property 700 750 800850 900 950 Particle Size Distribution, μm D₉₀ 45.4 47.7 58.0 52.9 49.244.5 D₅₀ 18.5 20.0 21.9 20.3 17.2 16.4 D₁₀ 3.2 3.6 4.2 3.2 3.1 3.3Fisher Sub-Sieve Size, 2.7 3.1 3.2 3.1 2.9 (3.5) μm Bulk density, g/cm³1.15 1.16 1.13 1.10 1.10 1.06 Oxygen Content, ppm 3400 4500 1100 11001050 980 Specific Surface Area, 4.97 4.08 1.89 1.06 0.7 0.57 m²/gCalculated Particle 0.13 0.16 0.34 0.61 0.93 1.13 Diameter, μm

[0068] As a rule, powder agglomeration due to sintering increases withthe reduction temperature. The test results demonstrate that, within abroad range of reduction temperatures, the size of the reduced Mo—Cucomposite powder and its bulk density do not increase monotonically withtemperature. Furthermore, there appears to be a pronounced sinteringeffect with increased temperature which manifests itself in a decreasein the specific surface area and, correspondingly, an increase of thecalculated particle diameter (the BET particle size). Similarly, theoxygen content follows the trend of the surface area and decreases withtemperature.

[0069] The bulk as-reduced Mo—Cu composite powders had a gray colorsimilar to unalloyed Mo powders. There was no visual indication of thepresence of Cu in the Mo—Cu composite powders. In order to examine thedistribution of phases in the composite powders, they were analyzed bySputtered Neutral Mass Spectrometry (SNMS) and cross-sectionedmetallographic samples were analyzed by Scanning Electron Microscopy(SEM) using Secondary Electron Imaging (SEI) and Back-Scattered ElectronImaging (BEI).

[0070] The Mo—Cu composite powders of this invention were shown toconsist of larger agglomerates of finite dual-phase particles comprisedof a sintered molybdenum network wherein the voids in the network arefilled with copper. This unique distribution of phases resulted in thesubstantial encapsulation of the copper phase by the molybdenum phase.As shown in the SEM micrographs, the finite particles were irregularlyshaped and ranged in size from about 0.5 to about 1.5 μm. This is inrelative agreement with the particle size calculated from the BETsurface area. FIG. 3 (BEI) is an enlarged image of the finite particleoutlined in the agglomerate shown in FIG. 2 (BEI) and demonstrates theencapsulation of the copper phase by the molybdenum phase. The SNMS testresults are consistent with the SEM observations in that they showdepletion of copper at the surface of the composite powder particles anda very homogeneous distribution of phases within the powder.

EXAMPLE 9

[0071] Production scale tests were carried out using a two-stagereduction of a synthesized CuMoO₄-based composite oxide having arelative copper content of 15 wt. %. The first stage reduction was donein the same furnace as in Example 6 using the same loading conditionsand a reduction temperature of 300° C. The end product was screened −60mesh and subjected to the second-stage reduction in a three-zonehydrogen reduction furnace having a temperature of 900° C. in all threezones. An oxide load of about 300 g produced a bed depth of about ½″ inan Inconel boat. The residence time for the material in the hot zone ofthe furnace was about 4 hours. After removal from the furnace, thereduced powder was immediately dumped for surface passivation into astainless steel hopper having a nitrogen atmosphere. Surface passivationfor 1 to 2 hours completely eliminated the pyrophoricity of the powder.The resulting Mo—Cu composite powder was screened −60 mesh and shown tohave the following properties:

[0072] Particle Size Distribution: D₉₀=47.0 μm

[0073] D₅₀=17.9 μm

[0074] D₁₀=2.8 μm

[0075] Fisher Sub-Sieve Size 2.9 μm

[0076] Specific Surface Area: 1.65 m²/g

[0077] Calculated Particle Diameter: 0.39 μm

[0078] Oxygen Content: 2700 ppm

[0079] Bulk Density: 1.26 g/cm³

[0080] Copper Content: 15.06 wt. %

[0081] The size of the powder made in this Example and the powder madeat 900° C. in Example 8 are very similar. However, the surface area andoxygen content are substantially higher while the BET particle size issubstantially lower. This indicates that the finite particles formed inthis Example are smaller than those formed in Example 8.

Consolidation of the Mo—Cu Composite Powders

[0082] The formation of a rigid Mo skeleton during solid-state sinteringis beneficial to obtaining good dimensional stability of Mo—Cupseudoalloy parts made by P/M. Mo—Cu pseudoalloys with a strong Moskeleton resist distortion during densification even in the presence oflarge amounts of liquid copper. High dimensional tolerances and anabsence of distortion are of particular importance for the P/M net-shapemanufacturing of thermal management components (heat sinks) formicroelectronic and optoelectronic applications.

[0083] In contrast to the Mo—Cu composite powders of this invention, thesintering of mechanical blends of elemental Mo and Cu powders issluggish. High sintering temperatures (up to 1650-1670° C.) are requiredto sinter the P/M compacts from blended metal powders which lead to theloss of copper in the form of bleedout and evaporation from the parts.The loss of copper makes it very difficult to achieve sintered densitiesabove 97% of theoretical density (TD). The use of sintering aids (Fe,Co, Ni) to improve the sinterability of such elemental powder blends ishighly undesirable as the thermal conductivity of Mo—Cu pseudoalloys isdramatically reduced.

[0084] For the Mo—Cu composite powders of this invention, we found thatthe copper content and distribution of Mo and Cu phases stronglyinfluenced the sintering conditions of the powder compacts. An inverserelationship was observed between the copper content and the sinteringtemperature of the compact.

[0085] In particular, sintering temperatures were found to extend fromthe solid-state sintering region of 1050-1080° C. for compacts having acopper content in the range of 26-40 wt. % to the region of sintering inthe presence of liquid copper at 1085-1400° C. for compacts having acopper content in the range of 2-25 wt. %.

[0086] Sintering in the presence of liquid copper included two stepswhich mimic the conventional infiltration method, viz., in-situsintering of a molybdenum skeleton and internal infiltration of theskeleton with liquid copper. Upon the melting of copper at 1083° C., themolybdenum skeleton is internally infiltrated with liquid copper viacapillary infiltration. The liquid copper is retained within themolybdenum skeleton by capillary pressure. Dissolved oxygen is removedfrom the molten copper at 1085-1100° C. The molybdenum skeleton isfurther sintered in the presence of the liquid copper to complete thedensification of the pseudoalloy.

[0087] The Mo—Cu composite powders may be consolidated in as-reduced,deagglomerated, or spray-dried flowable states. A lubricant and/orbinder may be mixed with the powder, or added during spray drying, toenhance powder consolidation. These materials may include for examplezinc stearate, ethylene-bis-stearamide, or ethylene glycol. The Mo—Cucomposite powders may be used in a number of conventional P/Mconsolidation methods such as mechanical or isostatic pressing,injection molding, tape forming, rolling, and screen printing forceramic metallization.

[0088] The following are the preferred processing steps for dewaxing andsintering green compacts made from the Mo—Cu composite powders of thisinvention:

[0089] 1. Depending on the type of wax/binder, dewax/debind the greencompacts at a temperature from about 200° C. to about 450° C.;

[0090] 2. Remove oxygen from the green compacts at a temperature fromabout 930° C. to about 960° C.;

[0091] 3. Sinter a substantially dense molybdenum skeleton at atemperature from about 1030° C. to about 1050° C.;

[0092] 4(a). Solid-state sinter compacts with a copper content of about26 wt. % to about 40 wt. % at a temperature from about 1050° C. to about1080° C.; or

[0093] 4(b). Sinter compacts with a copper content of about 2 wt. % toabout 25 wt. % in the presence of a liquid phase at a temperature fromabout 1085° C. to about 1400° C.

[0094] The Mo—Cu pseudoalloy shapes produced according to this methodexhibited no copper bleedout, very good shape retention, a high sintereddensity (about 97% to about 99%TD), and a fine pseudoalloymicrostructure (Mo grains in the range of about 1 μm to about 5 μm;copper pools in the range of about 2 μm to about 15 μm).

EXAMPLE 10

[0095] Mo-15Cu pseudoalloy samples were made from the Mo—Cu compositepowder (15 wt. % Cu) made in Example 9. To enhance consolidation, thepowder was blended with 0.5 wt. % ethylene-bis-stearamide, a solidlubricant made under the trade name of Acrawax C by Lonza, Inc. in FairLawn, N.J. The powder was mechanically pressed at 70 ksi into flatsamples (33.78×33.78×1.62 mm) having a green density of about 62% TD. Toassure uniform heat transfer to samples during dewaxing and sintering,the samples were processed in pure alumina sand. Thermal processing wasdone in flowing hydrogen in a laboratory furnace with an alumina tube.To prevent cracking of the tube by thermal stresses, the heating/coolingrate was limited to 2° C./min. The sintering cycle included: 1-hourisothermal holds at 450 and 950° C. for removing the powder lubricantand surface oxygen; a 1-hour isothermal hold at 1040° C. for in-situsintering of a molybdenum skeleton; a 2-hour isothermal hold at 1100° C.for internal infiltration of the skeleton with liquid copper, removal ofdissolved oxygen from molten copper, and presintering the samples; and a2-hour isothermal hold at 1230° C. for final densification of thesamples. The latter temperature was experimentally determined on thebasis of obtaining the highest pseudoalloy density without causingcopper bleedout by oversintering the molybdenum skeleton.

[0096] In several consecutive runs (3 samples per run), the as-reducedMo-15Cu powder demonstrated very good sinterability, an absence ofcopper bleedout, and good shape retention of the sintered compacts. Theaverage linear shrinkage was 15%, and the average values of the sintereddensity and electrical conductivity were in the range of,correspondingly, 98.8-99.0% TD and 36.6-36.7% IACS.

[0097] The thermal conductivity of the sintered samples was determinedfrom reported correlations between the electrical and thermalconductivity in Mo—Cu pseudoalloys. For an infiltrated Mo-15Cupseudoalloy, an electrical resistivity of 51.0 nΩ.m (equivalent to anelectrical conductivity of 33.8 %IACS) corresponds to a thermalconductivity of 166 W/m.K. A measured 1.085X increase in electricalconductivity for the samples made from the Mo-15Cu composite powderraised the thermal conductivity of the samples to a substantially higherlevel of about 180 W/m.K.

[0098] An SEM micrograph of a cross section of one of the Mo-15Cupseudoalloy samples is shown in FIG. 4. The molybdenum skeleton of thepseudoalloy is formed by mostly rounded, highly interconnected grainswhose distribution, order and size have been affected by regrouping andlimited growth in the presence of the liquid phase. The size of thegrains is in the range of about 1 to about 5 microns. Roundedinterconnected grains are indicative of a sintering mechanism consistingof particle rearrangement in the presence of a liquid phase and grainshape accommodation aided by the minute solubility of molybdenum inliquid copper at the sintering temperature. The average size of the Cupools is in the range of about 2 to about 15 microns. Deagglomeration ofthe as-reduced powder before sintering is expected to substantiallyimprove the microstructural homogeneity of the P/M pseudoalloy.

EXAMPLE 11

[0099] Mo-40Cu pseudoalloy samples were made from the Mo—Cu compositepowder (40 wt. % Cu) made in Example 7. Samples were pressed using thesame conditions as in Example 10. The higher copper contentsubstantially improved the pressibility of samples which exhibited agreen density of 73% TD. As in Example 10, the temperature for the finaldensification was experimentally determined on the basis of obtainingthe highest pseudoalloy density without causing copper bleedout byoversintering the molybdenum skeleton. It was established that the highcopper content limited the final densification temperature to 1065° C.thus bringing it into the solid-state sintering region.

[0100] In two consecutive runs (3 samples per run), the as-reducedMo-40Cu powder demonstrated very good sinterability and shape retentionof the sintered compacts. The average linear shrinkage was 9%, and theaverage values of the sintered density and electrical conductivity werein the range of, correspondingly, 97.8-97.9% TD and 50.7-51.0% IACS. Thelower linear shrinkage compared to that for Mo-15Cu samples in Example10 can be explained by the fact that the Mo-40Cu samples were pressed toa higher green density and consolidated to a lower sintered density.

[0101] An SEM micrograph of a cross section of a Mo-40Cu pseudoalloysample is shown in FIG. 5. By comparing the micrographs in FIGS. 4 and5, a dramatic difference between the solid-state sintering and sinteringin the presence of liquid phase becomes evident. The molybdenumskeleton, that has been sintered in-situ at 1040° C., has only slightlychanged during sintering at 1065° C. The clusters of Mo particles, whosesize and geometry have been barely affected by sintering, are indicativeof the absence of the particle rearrangement and the size accommodationsintering mechanisms that are operational only in the presence of aliquid phase. Correspondingly, the microstructure of the solid-statesintered pseudoalloy is less orderly (more clusters of Mo particles,larger Cu pools) than the microstructure of the pseudoalloy sintered inthe presence of the liquid phase. However, the high sintered density ofthe solid-state sintered material indicates that deagglomeration of theas-reduced Mo-40Cu powder before sintering may substantially improve themicrostructural homogeneity of the P/M pseudoalloy.

[0102] While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A molybdenum-copper composite powder comprising individualfinite particles each having a copper phase and a molybdenum phasewherein the molybdenum phase substantially encapsulates the copperphase.
 2. The composite powder of claim 1 wherein the individualparticles have a size of about 0.5 μm to about 1.5 μm.
 3. The compositepowder of claim 2 wherein the composite powder comprises agglomerates ofthe finite particles.
 4. The composite powder of claim 3 wherein theagglomerates have a size of about 15 μm to about 25 μm.
 5. The compositepowder of claim 1 wherein the powder contains from about 2 wt. % toabout 40 wt. % copper.
 6. A molybdenum-copper composite powdercomprising individual finite particles each having a sintered molybdenumnetwork wherein the voids in the network are filled with copper.
 7. Thecomposite powder of claim 6 wherein the powder has the color ofunalloyed molybdenum powder.
 8. The composite powder of claim 6 whereinthe individual finite particles have a size of about 0.5 μm to about 1.5μm.
 9. The composite powder of claim 6 wherein the powder contains fromabout 2 wt. % to about 40 wt. % copper.
 10. A method of making aCuMoO₄-based composite oxide powder comprising: (a) forming a mixture ofa molybdenum oxide and a copper oxide, the molybdenum oxide beingselected from ammonium dimolybdate, ammonium paramolybdate, ormolybdenum dioxide; and (b) firing the mixture at a temperature and fora time sufficient to form the CuMoO₄-based composite oxide.
 11. Themethod of claim 10 wherein a stoichiometric excess of up to 4 wt. %copper oxide is added to the mixture.
 12. The method of claim 10 whereinthe copper oxide is selected from cuprous oxide or cupric oxide.
 13. Themethod of claim 10 wherein the CuMoO₄-based composite oxide has ageneral formula of CuMoO₄+xMoO₃ where x is from about 29 to
 0. 14. Themethod of claim 10 wherein the mixture is fired at a temperature fromabout 650° C. to about 750° C. for about 5 hours.
 15. The method ofclaim 14 wherein a stoichiometric excess of up to 4 wt. % copper oxideis added to the mixture.
 16. The method of claim 15 wherein the copperoxide is selected from cuprous oxide or cupric oxide.
 17. The method ofclaim 14 wherein the CuMoO₄-based composite oxide has a general formulaof CuMoO₄+xMoO₃ where x is from about 29 to
 0. 18. A method of making aMo—Cu composite powder comprising: (a) reducing a CuMoO₄-based compositeoxide powder in a first stage to form an intimate mixture of metalliccopper and molybdenum oxides without the formation of low-melting-pointcuprous molybdate phases; and (b) reducing the intimate mixture in asecond stage at a temperature and for a time sufficient to reduce themolybdenum oxides to molybdenum metal.
 19. The method of claim 18wherein the first stage reduction is performed at a temperature fromabout 250° C. to about 400° C.
 20. The method of claim 19 wherein thesecond stage reduction is performed at a temperature from about 700° C.to about 950° C.
 21. The method of claim 18 wherein thelow-melting-point cuprous molybdate phases are Cu₆Mo₄O₁₅ and Cu₂Mo₃O₁₀.22. The method of claim 18 wherein the Mo—Cu composite powder ispassivated in nitrogen after the second stage reduction.
 23. A methodfor making a Mo—Cu pseudoalloy comprising: (a) consolidating a Mo—Cucomposite powder to form a compact, the Mo—Cu composite powder having acopper content from about 2 wt. % to about 40 wt. % and comprisingindividual finite particles each having a copper phase and a molybdenumphase wherein the molybdenum phase substantially encapsulates the copperphase; (b) sintering the compact in a first sintering stage at atemperature from about 1030° C. to about 1050° C. to form a molybdenumskeleton; (c) sintering the compact in a second sintering stage at atemperature from about 1050° C. to about 1080° C. for a compact madefrom a composite powder having a copper content of about 26 wt. % toabout 40 wt. %, or at a temperature from about 1085° C. to about 1400°C. for a compact made from a composite powder having a copper content ofabout 2 wt. % to about 25 wt. %.
 24. The method of claim 23 wherein theMo—Cu composite powder is combined with a binder and/or lubricant priorto consolidation.
 25. The method of claim 24 wherein the compact isheated at a temperature from about 200° C. to about 450° C. beforesintering to remove the binder and/or lubricant.
 26. The method of claim23 wherein the compact is heated at a temperature from about 930° C. toabout 960° C. to remove oxygen before sintering.
 27. The method of claim23 wherein the Mo—Cu pseudoalloy has a density of about 97% to about 99%theoretical density.
 28. The method of claim 27 wherein the Mo—Cupseudoalloy has a microstructure having molybdenum grains in the rangeof about 1 μm to about 5 μm and copper pools in the range of about 2 μmto about 15 μm.